1. Field of the Invention
This invention relates to flame-resistant polyimide foam insulation prepared from polyisocyanates and aromatic polycarboxylic compounds.
2. Description of the Prior Art
The preparation of polyimide foams from polyisocyanates and aromatic polycarboxylic acid derivatives has generally been accomplished heretofore in the presence of various catalysts and with suitable heating. Rosser, for example, teaches in U.S. Pat. No. 3,772,216 that such foams may be prepared at temperatures of about 90.degree. to 300.degree. C. in the presence of an alkanolamine. While the products thus obtained possess excellent fire-retarding properties, their application has had to be restricted to situations where the required heating is practical. Other processes have been disclosed by which this type of foam can be prepared without recourse to external heat, thus allowing the material to be formed in situ, e.g. between walls, on walls or in similar places. This has been accomplished by the use of catalysts of a tertiary amine and an aliphatic alcohol containing one to six carbon atoms (McLaughlin, U.S. Pat. No. 3,620,987) or a monomeric homocyclic polyepoxide (Grieve, U.S. Pat. No. 3,644,234).
More recently, tertiary amines have been used with furfuryl alcohol as catalyst for condensing a polyisocyanate with a suitable active-hydrogen compound to obtain foam containing carbodiimide and isocyanurate linkages (Narayan et al, U.S. Pat. No. 3,894,972.
Strong organic acids such as formic acid and chloroacetic acid, have been used with aromatic isocyanates and carboxylic acids to produce amide rather than urea linkages (Phillips et al, "Polyurethanes", 1964, pages 108-110). Sulfuric acid, interalia, has been used with teriary alcohols and polycarboxylic acids to form an intermediate alkyd resin which is then foamed with a polyisocyanate (Hindersinn et al, U.S. Pat. No. 2,865,869). It is interesting to note that this process is said to greatly reduce the heat normally liberated during foaming and thus prevent the charring which often occurs in lightweight foams. La Spina et al., (U.S. Pat. No. 3,931,059), have disclosed the preparation of polycarbodiimide foams from a polyisocyanate, using a pH 1 to 8 protonic acid and an alcohol in the presence of a phosphine oxide or sulfide catalyst, while Stierling, on the other hand, formed expanded-in-place cellular bodies from a thermosetting phenolaldehyde resin and an exothermic mixture of hydrogen peroxide and, e.g., sulfuric acid or phosphoric acid.
The object of the present invention is to provide a simple quick process to form polyimide foams in situ from components which have shown a tendency to char when brought together in the presence of a strong acid. Another object is to obtain low density cellular products that have excellent flame resistance and insulation properties.